Frequency synthesizers are well known in the art and are commonly used to generate reference signals in communication devices. Frequency synthesizers are available in a variety of topologies with varying scope of performance. Some topologies use a single loop phase locked loop to provide a closed loop and thereby accurately lock to a desired signal. Others use multiple phase locked loops to lock to a desired channel with more accuracy and with less noise. The latter has been used in more sophisticated synthesizer circuits with more stringent requirements. With ever more increasing demands on communication devices to accommodate more users, the need to more efficiently utilize the available frequency spectrum increases. This has been accomplished by squeezing more channels in the operating band by placing them closer to each other. Some of the problems with narrow channel spacing are spurious noise and frequency accuracy. Both of which are present in all systems and must be sufficiently controlled prior to successful communication. However, due to the closer channel spacing, spurs must be sufficiently attenuated at frequencies close to the carrier. Another approach of allowing more users to operate in a particular portion of the frequency spectrum has been the use of time division multiplexing (TDM). The channel spacing in TDM systems may also be required to be narrow. Furthermore, these systems use fast lock synthesizers to provide them with the required fast frequency switching.
Multi-loop synthesizers are good candidates to combat the problems observed in the aforementioned systems. A variety of multi-loop synthesizers are available using a multiple of phase locked loops in series. In some applications, each loop is used as the reference signal generator for the next loop. A problem with this approach is that exact frequencies can not always be achieved due to the nature of integer dividers that are used with the loops. No significant spurious response improvement can be realized with these synthesizers. Furthermore, the synthesizer settling (lock) time is not improved over the single loop synthesizer due to the need for narrow band filters. It is clear that this type of multi-loop synthesizer while attempting to solve one type of problem is inadequate to solve another. It is therefore clear that a need exists for a synthesizer that achieves low spurious response and which is able to fast lock to any desired frequency.